Conventionally, in order to prevent entry of foreign matter into a space of a counterbore hole or a hole of a bolt head, by flattening the position of the counterbore hole or the hole of the bolt head of a socket head bolt, with the bolt loaded so that the bolt head may sink into the inside of the counterbore hole, it has been proposed to fill the space of the counterbore hole or the hole of the bolt head with a filling member (including plug member) (see Japanese Patent Application No. 8-334121 and Japanese Patent Application No. 11-37127).
On the other hand, when the space of the counterbore hole or the hole of the bolt head is filled with a filling member in a tight state, entry of foreign matter can be prevented, but at the same time, for example, in the case of a grinder having a cutter blade mounted on a rotating element main body by a bolt so as to be exchangeable, it is difficult to repair or replace the members, and it is time-consuming.
To the contrary, when filled with the filling member in a loose state, the filling member may be detached during use, and the initial objective of prevention of entry of foreign matter may not be achieved.